The United States Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00024-94-D-5204 awarded by the United States Navy.
1. Field of the Invention
The invention relates generally to high-speed coordinate scan conversion of radar coordinates for video presentation and more particularly to software programmed video coordinate conversion.
2. Description of the Background
Conventional radar displays consist of circular-shaped cathode ray tubes (CRT) in which a rotating beam is made visible by electron bombardment of a phosphor coating on a CRT screen. Targets can be identified on the screen as the beam rotates in synchronism with a rotating antenna. This type of display is known as a Plan Position Indicator (PPI).
A PPI display has a number of objectionable characteristics. Because of the fact that it relies on the persistence of a phosphor, there is an inherent lack of brightness. Thus, the early tubes could be viewed satisfactorily only under very low levels of ambient lighting, and refreshing of the PPI display occurred only once per revolution of the radar antenna, and it, therefore, was dependent on the radar revolution rate.
In order to overcome these deficiencies and to achieve other advantages, scan converters have been developed to convert the PPI information, which is a function of the radius (r) and the angle (.theta.) of the radial beam from a reference location to TV or computer screen monitors in which the (x) and (y) coordinates of the screen are used to determine the image. Scan converter systems allow for the integration of radar displays and with computer video recording techniques, including multiple color displays, overlapping windows and the capability of adding text to the display.
Numerous types of such systems have been developed for providing the conversion of the (r,.theta.) information into the (x,y) information. The great majority of these relied on relatively complex hardware-dominated systems for providing the scan conversion. In the past such complex hardware systems were required to achieve the fast speed needed to process the real-time information being received from the radar return.
Software algorithms for radar coordinate digital scan conversion have been developed, as shown in U.S. Pat. No. 4,697,185 entitled "Algorithm for Radar Coordinate Conversion and Digital Scan Converters," issued Sep. 29, 1987 to David M. Thomas et al., and U.S. Pat. No. 4,931,801 entitled "Method and Apparatus to Scan Convert Radar Video to Television Outputs," issued Jun. 5, 1990 to William R. Hancock. These algorithms were joined with specialized hardware to provide the desired (r,.theta.) to (x,y) scan conversion.
In the Thomas et al. patent it was noted that near the center or origin of a PPI display, the azimuthal resolution of the radar is greater than the resolution of the display, and, therefore, a number of (r,.theta.) points must be matched to the same (x,y) point. At long ranges in a PPI display, however, the radar resolution will often be less than that of the display. This results in a number of open areas in the display which have to be filled in. At intermediate ranges, the resolution of the radar and the display are approximately equal, and there may be a one-to-one mapping between the two coordinate systems.
In the Thomas et al. patent, look-up tables are utilized to hold sin and cos values to update the x and y values to the next consecutive coordinate of x and y values by adding a sin value to the x coordinate and a cos value to the y coordinate. In the Hancock patent look-up tables were also employed to control intensities of the display pixels. Look-up tables have also been employed in graphic displays to control colors of the image displayed.
A copending U.S. patent application Ser. No. 08/143,597, entitled "Programmed Radar Coordinate Scan Conversion" was filed on Nov. 1, 1993 and assigned to the assignee of this invention. The invention of this prior application was also directed to software programmed radar scan conversion. In the invention of this prior application, radar scan conversion from (r,.theta.) values employed in a PPI display are converted to (x,y) coordinates of a computer monitor by utilizing a digital computer which employs look-up tables, wherein the look-up tables are utilized in an algorithm which first computes an inverse mapping of the (x,y) coordinates of the monitor to the (r,.theta.) coordinates of the PPI display to fill the look-up table with values that link together the (x,y) points to the corresponding (r,.theta.) points.
During this mapping some of the (r,.theta.) points will not have been "hit" or converted. To complete the mapping process a second phase "forward mapping" is then performed which links the remaining (r,.theta.) coordinates which have not been mapped during the inverse mapping phase to (x,y) coordinates. Each table entry represents an image patch. The number of pixels in a patch varies according to the radial distance of the patch from the origin of the display to compensate for the differences between the resolution of the radar and the resolution of the display. Since the look-up table has been established, the algorithm relates the predefined patches to the coordinate points of the display.
In the present invention a process provides radar scan conversion from radar amplitude data in polar coordinates to rectangular coordinates by a digital computer which receives (r,.theta.) coordinate amplitude data from a radar receiver and which supplies (x,y) coordinate amplitude data to a monitor display. A software program generates an aggregate radial scan pattern that consists of a plurality of radials each of which have active lengths that span one or more of a plurality of selected zones of the display such that as the average azimuthal resolution associated with each zone increases, the number of generated radials match the average azimuthal resolution of the display for each zone.
FIG. 2 illustrates how the zones of a four-zone display may be arranged to satisfactorily partition an aggregate radial pattern. The segment shown in FIG. 2 is not to scale and in fact occupies much less of the screen 32 that is shown in FIG. 2 in order to illustrate the desired pattern with sufficient clarity. The scan pattern 30 may be divided into 0.1.degree. angular segments or other suitable divisions so the segment is actually much smaller than is represented in FIG. 2.
Each segment has 8 radial or partial radials which form the aggregate radials. One radial of each segment 30 is radial 42 that is active through all four zones, region 0, region 1, region 2 and region 3. The zone is bisected in the embodiment shown in the FIG. 2, which is just one of a number of alternative ways the invention might be implemented. The radial that bisects scan pattern 30 is the radial 44 which extends through all of the ring zones, but not the central circular zone, or region 0.
There are two radials, 46 and 48, which bisect the portion of segment 30 between the radial 42 and the radial 44 and between the radial 44 and the radial 56, respectively, where the radial 50 is the start of the next segment when the scan rotates in a clockwise direction. The desired resolution of regions 0-2 is completed with the radials described thus far. The radials 52, 54, 56 and 58 bisect the remaining areas between the radials 42 and 46, 46 and 44, 44 and 48, 48 and 50, respectively, in order to complete the scan pattern of the outermost zone. In this manner, the number of scans in the aggregate scan are divided so that for each segment of the scan, zone 0 has 1 radial, zone 1 has 2 radials, zone 2 has 4 radials and zone 3 has 8 radials, thereby allowing the number of radials in each zone to match the average azimuthal resolution of the display in the zone mentioned. The circular center zone 0 desirably has the same azimuthal resolution of the radar, or as a pulse expanded or compressed version of the received radar pulses. The number of radials in each zone is desirably doubled in each zone, but other ratios may be employed, with or without pulse expansion or compression.